1. Field
Aspects of the present invention generally relate to electrodynamic machines, and a method for reducing vibration of an electrodynamic machine.
2. Description of the Related Art
Machines with rotating masses, including electrodynamic machines, inherently have critical vibration excitation speeds attributable to the structural mechanics of the device and associated system operational forces. For example, the structure and bearing housings of electrodynamic machines are exposed to different mechanical forces, i.e., unbalanced forces, and electromagnetic forces, i.e., generation of electromagnetic fields, which manifest as vibrations. Whenever excitation frequencies coincide with the natural frequencies of the structures, high vibration occurs. Exposure to such high vibration can damage or cause catastrophic breakdown of the machinery. Hence, high vibration is undesirable in the machinery. It is known to design structures and/or components of electrodynamic machines with low structural vibration, wherein the natural frequencies of the system and/or components are very different compared to operating frequencies and speeds.
Induction motor vibration influences that ultimately contribute to motor critical vibration speed include among other things: rotor length to diameter ratio, rotor core cross-sectional structure, shrink fit pressure between the rotor core and shaft, stacking spacing between adjoining rotor core laminas, changes in alternating current excitation frequency established by variable speed motor drive controllers, and oil whip rotor-dynamic stability induced by the hydrodynamic bearings that support the rotor shaft. With respect to excitation frequency vibration influences, induction motors generally are optimized for 50 Hz or 60 Hz alternating current excitation frequencies, including any operational vibration responses. However, AC induction motors that are coupled to variable speed motor control drives, for example a variable frequency drive (VFD), often vary the AC excitation frequencies in a range from 30 Hz to 75 Hz. This increased range of AC variable excitation frequencies increase proportionally the motor's responsive critical vibration frequency range. Thus, a need exists for an electrodynamic machine, for example an induction motor, operated with a variable frequency drive which comprises reduced structural vibrations.